Roof/Wall Connection on a Pretty Good house, Zone 5A, Cold 6000HDD
Hello everybody. I’ve been perusing GBA for a while trying to come up with a wall for this 2 story, ~2000SF house that makes sense, is not too expensive, and is durable. I’m trying to avoid using foam unless absolutely necessary. In the attached drawing you will see my working idea for my roof and wall at the downward slope of what will be a shed roof (1 1/2″ / 12″ slope).
The basic build up of the wall is
- Painted GWB or Finish plywood (at some locations)
- 2×6 framed walls with dense pack cellulose
- zip sheathing, taped at exterior serving as air barrier and WRB
- 2 layers of 3″ Roxul Comfortboard 80 (6″ total)
- 1x strapping
- standing seam metal or wood siding, depending on wall
The roof is the same, except replace 2×6 framing with 14″ LPI18 (like a TJI)
I have indicated on the drawing what I think is the air barrier (taped zip sheathing).
This assembly gives a total of ~R43 at walls and ~R73 at roof. I am in the Hudson Valley (Cold 5A, ~6000 HDD).
My concerns are the following.
1. Am I doing something horribly wrong? Do I need some kind of venting?
2. Do I have a danger of cold sheathing?
3. Am I overinsulating? Underinsulating?
4. Is it wrong to use Zip+tape as my WRB/Air Barrier? Are there better solutions?
I think I saw someone who installed Zip w/ membrane on the inside chord of his roof craming… maybe using wood fiberboard as exterior sheathing at roof. That is an idea I’ve considered, to avoid condensation on outboard roof sheathing, but I’m trying to minimize different systems/labor costs…
Some of my thoughts… Though I think I could save some $ by going to 5″ Rock wool, I think having 2″ and 3″ boards on site will be confusing and not worth the hassle, so might as well go with the full 6″. I feel like 4″ of rock wool is cutting it too close in terns of cold sheathing. Perhaps I should eliminate zip and add WRB/Air barrier as a wrap outside of the rockwool?